Senna bicapsularis aka Rambling Cassia

Senna bicapsularis is a shrub in the pea family, Fabaceae. It also has an older name, Cassia bicapsularis. Gardeners call it butterfly bush or winter cassia. It comes from the tropics of the Americas. Its range includes Central America and the Caribbean. In South America, it grows in Colombia and Venezuela. Its range also includes Ecuador and Peru. It grows in thickets, at woodland edges, and along streams. People grow it as a hedge in the tropics. It has spread from gardens and taken hold in some places.

In gardens, it grows 8–12 feet (2.4–3.7 metres) tall. It spreads to 8–10 feet (2.4–3.0 metres) wide. Reports from the tropics give heights up to 39 feet (12 metres). The shrub has an open, uneven crown and several stems. Its branches droop, sprawl, and may lean on nearby shrubs. Garden reports call it semi-evergreen, meaning it keeps some leaves. Reports from the tropics also note leaf loss. Its bark is green to light brown. Its pinnate leaves have three to five pairs of leaflets. These pairs grow along a shared leaf stalk. Each smooth, dark green leaflet has an unbroken edge. The leaves can have a strong, bad smell.

Yellow flowers grow in loose groups through fall and winter. Their colour ranges from pale yellow to deep gold. Each flower forms a cup, with curved stamens that stand out. Thin brown pods follow the flowers. They reach 0.5 feet (0.15 metres) long. Bees visit the flowers, and insects pollinate them. The cloudless sulphur butterfly uses this plant as a host. Its caterpillars eat leaves and flower buds in fall. Their feeding can leave fewer flowers to show.

Full sun gives the best growth and bloom. The shrub grows in clay, sand, or loam that drains well. It copes with acid soil and soil that is slightly alkaline. It can bear some drought and exposure to salt. Once settled in, plants need some water during dry spells. Sources place it in USDA zones 9–11. Frost can kill stems, but live roots may grow back. The whole plant may die if its roots freeze. Cut frost-killed stems to ground level before spring. Pinch young shoots through early September to spur branches and flowers.

Use it in borders, pots, or as a small specimen tree. Nearby shrubs can help hold up its weak stems. Tree forms may need stakes, and branches can break. Prune after bloom to help keep sprawling growth in check. Sources report few major problems with pests or disease. Useful Tropical Plants lists sweet pulp in the pods as edible. It also reports cooked leaves and roasted seeds used in place of coffee. That source lists no known hazards, but this does not prove safety. It records past use of leaves as remedies, including to purge the bowels. Its hard wood has dark heartwood and yellow sapwood.